With the emphasis on the development of alternative forms of energy, solar cells that capture solar energy and convert the solar energy into electrical energy are in demand with increased emphasis being placed on the efficiency and cost effectiveness of the solar cells. One type of solar cell that offers a number of advantages, including conversion efficiency, is an IMM solar cell. An IMM solar cell includes a plurality of subcells having band gaps that are selected to efficiently capture the solar energy. In order to appropriately tailor the band gaps, however, the subcells may be formed of materials that are lattice mismatched and/or that are coefficient of thermal expansion (CTE) mismatched. In an effort to reduce the negative impact of the lattice and/or CTE mismatch and as their name suggests, IMM solar cells are fabricated, not from the bottom up as in a conventional semiconductor fabrication process, but by growing the uppermost subcell initially followed by the intermediate subcell and finally the lowermost subcell. By fabricating an IMM solar cell in this sequence, the deleterious effects of the lattice and CTE mismatch may be concentrated within the lowermost subcell so as to have less impact upon the performance of the IMM solar cell.
One type of IMM solar cell includes a lowermost subcell formed of indium gallium arsenide (InGaAs). By tailoring the percentage of indium included within the InGaAs subcell, the band gap of the InGaAs subcell may be reduced to approximately the optimal energy band gap for solar energy collection of about 1.0 eV. However, formation of the lowermost subcell from InGaAs may create a lattice and CTE mismatch relative to the other subcells, thereby limiting the performance of the resulting IMM solar cell.
IMM solar cells are generally relatively thin and may, for example, include subcells having a collective thickness of about 10 microns. In order to permit the IMM solar cells to be fabricated and otherwise handled without an excessive amount of breakage, the IMM solar cells may be mounted upon a carrier. For example, the carrier may be formed of germanium (Ge), glass, ceramic or other material that is bonded to the IMM solar cell with an adhesive, such as an room temperature vulcanizing (RTV) adhesive. While a carrier facilitates the handling of an IMM solar cell, the carrier increases the cost of the IMM solar cell structure, that is, the IMM solar cell in combination with the mechanical carrier. Additionally, the carrier is typically disadvantageously thermally mismatched relative to the IMM solar cell.
As such, it would be desirable to design an improved IMM solar cell. In particular, it would be desirable to provide an IMM solar cell containing subcells with performance that is less limited by lattice and/or CTE mismatching.